1. Field of the Invention
This invention relates to an OFDM receiver for receiving an OFDM (Orthogonal Frequency Division Multiplexing) modulated high frequency signal by diversity synthesis, and particularly relates to an OFDM receiver suitably used in a television receiver for car mounting.
2. Description of the Related Art
FIG. 4 shows a conventional OFDM receiver. In FIG. 4, plural antennas (shown in the case of four antennas) 111, 121, 131, 141 for receiving an OFDM modulated high frequency signal are arranged in places separated from each other on a vehicle as one example. Receiving portions 112, 122, 132, 142 are correspondingly connected to the respective antennas 111 to 141. The respective receiving portions 112 to 142 have the same construction, and frequency-convert the high frequency signal to be received to an intermediate frequency signal. A/D converters 113, 123, 133, 143 are connected to the next stages of the respective receiving portions 112 to 142. Each A/D converter converts the intermediate frequency signal to a digital signal, and outputs a baseband signal of a time domain.
First to fourth OFDM demodulating means 114, 124, 134, 144 are connected to the next stages of the respective A/D converters 113 to 143. The respective OFDM demodulating means 114 to 144 have a high speed Fourier converter of the same construction therein, and perform conversion to the baseband signal of a frequency domain by performing Fourier transformation by taking synchronization of the baseband signal of the time domain.
A first phase control circuit 151 is connected between the output terminal of the first OFDM demodulating means 114 and the output terminal of the second OFDM demodulating means 124 among the above four OFDM demodulating means. A second phase control means 152 is connected between the output terminal of the first OFDM demodulating means 114 and the output terminal of the third OFDM demodulating means 134. A third phase control means 153 is connected between the output terminal of the first OFDM demodulating means 114 and the output terminal of the fourth OFDM demodulating means 144. A first phase shifter 154, a second phase shifter 155 and a third phase shifter 156 are respectively connected to the next stages of the second to fourth OFDM demodulating means.
The respective phase control means 151 to 153 have the same construction, and respectively compare the phase of the baseband signal of the frequency domain outputted from the second to fourth OFDM demodulating means 124, 134, 144 and the phase of the baseband signal of the frequency domain outputted from the first OFDM demodulating means 114, and respectively output its phase difference signals to the first to third phase shifters 154 to 156. Each of the phase shifters 154 to 156 outputs a baseband signal of the frequency domain conformed to the phase of the baseband signal of the frequency domain outputted from the first OFDM demodulating means 114 by changing the phase of the inputted baseband signal of the frequency domain by each phase difference signal.
The baseband signal of the frequency domain outputted from the first OFDM demodulating means 114 and the baseband signal of the frequency domain outputted from the first phase shifter 154 are added by a first adder 157. The baseband signal of the frequency domain outputted from the second phase shifter 155 and the baseband signal of the frequency domain outputted from the third phase shifter 156 are added by a second adder 158. A third adder 159 is connected between the output terminal of the first adder 157 and the output terminal of the second adder 158. Accordingly, the baseband signals of the frequency domain outputted from all the OFDM demodulating means 114 to 144 are finally added by the third adder 159 in the same phase relation. Accordingly, a baseband signal having maximum signal electric power is obtained from the third adder 159. A bit error included in the added baseband signal is corrected by an error correcting means 160, and the corrected baseband signal of the frequency domain is outputted.
When there is an antenna greatly reduced in level of the received high frequency signal by fading caused by e.g., the movement of a mounted vehicle in the conventional OFDM receiver, it is impossible to accurately take the synchronization of the baseband signal of the time domain in the OFDM demodulating means corresponding to this antenna. In such a state, no Fourier transformation can be also accurately performed by this OFDM demodulating means so that no baseband signal of electric power sufficient to correct the error and corresponding to the number of antennas can be obtained. Accordingly, the problem of causing a reduction in image quality, etc. is caused.
Further, since the expensive OFDM demodulating means is arranged by the same number correspondingly with each antenna, it has a disadvantage in that the receiver is high in cost.